Donald Peterson is an anxious teacher who has just moved to a new house with his pregnant wife Sarah Peterson. He accepts a teaching job at St. Bernadette's primary school, taking over the class formerly taught by Mr. Maddens, who has left for the United States; in the interim, the enthusiastic and childlike teaching assistant Mr. Poppy has been guiding the class on his own. The children dream of entering the competition called A Song for Christmas, a national contest where each school produces a Christmas song, with the winner earning £10,000 and a shot at becoming a Christmas number 1. Yet headteacher Mrs. Bevan refuses to authorize the entry without a qualified teacher and worries that her nephew, Mr. Poppy, may not be able to keep a steady hand on the reins.
Behind Donald’s quiet demeanor lies a weighty family dynamic. He lives in the shadow of his domineering father and his twin brother, world-famous composer and conductor Roderick Peterson, who is a prominent figure in the music world. When the decision to enter the competition is discussed, Mr. Poppy persuades Donald to sign the entry forms, and then, in a bold turn, abducts him for an impromptu road trip toward Castell Llawen, a fictional Wales locale home to the Christmas Castle where the show will take place.
Meanwhile, the rivalries and ambitions broaden. Roderick is competing on the other side, mentoring the choir of the prestigious St Cuthbert’s College, and a separate rivaling class under Mr. Shakespeare Mr. Shakespeare from Oakmoor School has its own entry. Donald and Mr. Poppy lead their class through the wild terrain of Wales, pushing through fatigue, rushing rivers, and even handling a baby smuggled into the group in the form of a tiny passenger, all while scaling a mountain to reach the castle in time for the performance.
When at last they reach the venue, a series of reconciliations and revelations unfold. Donald is reunited with Sarah Peterson, and Mr. Poppy is reunited with Mrs. Bevan. Yet the competition’s drama intensifies as Roderick, determined to claim victory, steals the baby and a portion of Mr. Shakespeare’s song, and in a bid to ruin the chance for Donald’s group, traps Mr. Poppy and the class inside a giant snow globe and disqualifies them. The resilience of teamwork, however, shines through as Mr. Shakespeare’s class and others step in to help, while Mrs. Bevan retrieves the baby and St. Bernadette’s delivers two heartfelt songs, even if they momentarily pretend to be Oakmoor in order to keep hope alive.
The story culminates in a dramatic and joyful turn of events. As they prepare to leave, Sarah goes into labor, and the family finds shelter in a barn where Donald assists in the birth of twins. The moment is intimate and triumphant, with the new babies signaling a fresh page for the family. Back at the castle, Oakmoor’s performance is declared the winner, though the members of St. Bernadette’s feel the show belongs to them, especially after the inspiring moments they shared. In the final act, Mr. Shakespeare and Roderick arrive at the barn, and as Mr. Poppy and the class sing another Christmas song for Donald, Sarah, and their newborn sons, the two brothers reach a quiet, meaningful reconciliation, deciding to bring the family together more often in the future.
